Heparan sulfate glycosaminoglycans (HSGAGs) comprise an important polysaccharide constituent of many proteoglycans (Bernfield et al., 1999 Annu Rev Biochem 68, 729-777). These glycans are linear polymers based on the variably repeating disaccharide unit (uronic acid α/β1→4 glucosamine)n, where n represents a variably repeating number (typically 10-200). As present in nature, these sugars possess an extensive chemical heterogeneity which is largely attributed to the mosaic arrangement of O- and N-linked sulfates present at different positions along each sugar chain (Esko et al., 2001 J Clin Invest 108, 169-173; Sasisekharan et al., 2002 Nat Rev Cancer 2, 521-528). Additional structural variations include the presence of N-linked acetates at the glucosamine C2 position as well as the epimerization of the uronic acid C5 carboxylate that distinguishes β-D-glucuronic acid from α-L-iduronic acid. Fundamental to understanding HSGAG structure-activity relationships is the appreciation that the polydispersity of glycan fine structure is not random. Instead, it is the end product of activities regulated in a cell and tissue specific fashion. This programmed diversity of HSGAG structure (Esko et al., 2001 J Clin Invest 108, 169-173) ultimately plays out at a functional level, namely through the dynamic regulation of numerous biochemical signaling pathways (Esko et al., 2001 J Clin Invest 108, 169-173) relating to such processes as cell growth and differentiation (Sasisekharan et al., 2002 Nat Rev Cancer 2, 521-528), cell death (Prince et al., 2002 Dev Dyn 223, 497-516; Lai et al., 2004 Gastroenterology 126, 231-248), intercellular communication, adhesion and tissue morphogenesis (Hacker et al., 2005 Nat Rev Mol Cell Biol 6, 530-541). HSGAGs (present as structurally-defined binding epitopes on the cell surface) also play an important role in microbial pathogenesis (Liu et al., 2002 J Biol Chem 277, 33456-33467; Vives et al., 2006 Curr Gene Ther 6, 35-44).
In contrast to the complex enzymatic process by which these polysaccharides are made, it appears that their catabolism is more straightforward, both in the scope of its purpose and the means by which it is carried out at the biochemical level. In the mammalian lysosome for example, GAG degradation follows an obligatory sequence of depolymerization steps, using enzymes which follow a predominantly exolytic mode of action. As such, the substrate specificity of one enzyme is largely predicated on the activity of the enzymes which precede it. Essential to this sequence are several sulfohydrolases which desulfate the sugar backbone as a prerequisite to the ensuing glycosidase step. These sulfatases are structure-specific enzymes, each one hydrolyzing a unique sulfate position within the heparin disaccharide repeat unit present at the non-reducing end.
Sequential GAG degradation is not unique to the eukaryotic lysosome. This process has been demonstrated in several microorganisms as well (Dietrich et al., 1973 J Biol Chem 248, 6408-6415; Nakamura et al., 1988 J Clin Microbiol 26, 1070-1071; Lohse et al., 1992 J Biol Chem 267, 24347-24355), which depend on sulfated polysaccharides not only as a carbon source but often as a means of scavenging inorganic sulfate (Kertesz, 2000 FEMS Microbiol Rev 24, 135-175). The gram-negative soil bacterium Flavobacterium heparinum (a.k.a. Pedobacter heparinus) is an excellent example of this process, having also proven to be a particularly rich biological source for the isolation and molecular cloning of several GAG-degrading enzymes (Sasishekaran et al., 1993 Proc Natl Acad Sci USA 90, 3660-3664; Godavarti et al., 1996 Biochem Biophys Res Commun 225, 751-758). Like the lysosomal pathway, many of the flavobacterial enzymes possess a high degree of substrate specificity.